1. Field of the Invention
The subject invention is directed to rotary vane pumps, and more particularly, to a balanced split discharge vane pump that provides a first discharge flow for high fluid demand conditions and a second discharge flow for low fluid demand conditions, and to a system for metering fluid flow from a split discharge vane pump depending upon fluid demand conditions.
2. Description of Related Art
Rotary hydraulic vane pumps are well known in the art, as disclosed for example in U.S. Pat. No. 4,274,817 to Sakamaki et al. and U.S. Pat. No. 5,064,363 to Hansen. A typical rotary vane pump includes a circular rotor mounted for rotation within a larger circular pumping chamber. The centers of these two circles are typically offset, causing eccentricity. Vanes are mounted to slide in and out of the rotor to create a plurality of volume chambers or vane buckets that perform the pumping work. On the intake side of the pump, the vane buckets increase in volume. These increasing volume vane buckets are filled with fluid that is forced into the pumping chamber by an inlet pressure. On the discharge side of the pump, the vane buckets decrease in volume, forcing pressurized fluid out of the pumping chamber.
It is desirable to match the fluid displacement of a vane pump to the operating characteristics of the system with which the pump is to be associated. For example, the maximum displacement of a fuel pump should be coordinated with the maximum fuel requirements of the associated engine application. However, system requirements typically vary with operating conditions, so that a fixed displacement fuel pump that is designed as a function of the most demanding engine operating conditions may function with less than desired efficiency under other operating conditions.
In the case of a fuel pump associated with a gas turbine engine of an aircraft, fuel flow requirements, as quantified by pump displacement per rotational speed, under engine starting conditions greatly exceed fuel flow requirements during other less demanding engine operating conditions, such as cruise, idle, decent and taxi. Various attempts have been made to improve fuel pump efficiency over the operating envelope of a gas turbine engine, by utilizing different valving arrangements at the pump outlet to meter a portion of the pump discharge to the engine as a function of engine demand, while recirculating the remainder of the flow back into the pump. However, these prior art arrangements are typically complex and thus add cost to the pumping system. In other implementations, variable displacement pumps have been utilized to match pump output flow to system demand. However, these implementations are at the expense of pump size/weight and reliability because of an increase in pump radial/axial loading and incorporation of additional moving parts.
It would be beneficial therefore to provide a positive displacement vane pump that is adapted and configured to more closely match the operating characteristics of the system with which it is associated, as well as a valving arrangement for effectively managing the flow of fluid from the pump depending upon the fluid demand conditions of the system with which it is associated. This is achieved by retaining the simple features of fixed displacement pumps and hence preserving their weight and reliability advantages.